Some information the university provided was already public, like government filings or published university policies, and some of it was made available only under highly restricted conditions, Mr. Grassley, Republican of Iowa, said in an interview. “We’re getting stonewalled,” he said. “We’re getting nowhere. We were promised cooperation and we’re not getting it.”

William A. Brewer III, an N.Y.U. trustee who is helping facilitate the university’s response, said that, to the contrary, the university had gladly complied with the inquiry and that “at a high level, all categories of information have been provided.”

The inquiry grew out of Jacob J. Lew’s confirmation hearings as Treasury secretary, during which he was questioned about the subsidized mortgages he had received from the university and a $685,000 parting bonus he received when he was its executive vice president. Even after Mr. Lew was confirmed by the Senate, Mr. Grassley ratcheted up his questioning of the university, saying he was doing so because of its nonprofit status, which affords it considerable income and property tax exemptions.

In a letter dated March 15, Mr. Grassley asked N.Y.U. to provide details about Mr. Lew’s compensation and other loans the university had made since 2000, as well as related university policies. He requested a reply by March 29.

On that date, the university wrote that it was “N.Y.U.’s intention to voluntarily respond to your request, with sufficient information to enable you to evaluate the policy issues.” That letter included Form 990s — tax documents that nonprofit organizations must make public each year — and information about N.Y.U.’s bylaws and compensation policies. “We will be providing other materials in response to your request in the near future,” the document continued. “As you might imagine, it will take some time to supply information responsive to all of your questions.”

A second letter, dated April 19, included various policies and guidelines, summaries of two loan programs, redacted board of trustees and compensation committee minutes and, among other readings, a Wall Street Journal opinion article. “N.Y.U. is continuing to collect and process additional responsive materials,” the letter said. “We expect to be back to you very shortly about your requests concerning Secretary Lew.”

A few weeks later, N.Y.U. representatives brought documents relating to Mr. Lew’s loans over to the Dirksen Senate Office Building, where two members of Mr. Grassley’s staff reviewed them. But though they were allowed to take notes, they were not permitted to make copies or to keep any of the documents for further review. Then on June 28, the university provided the total number and size of loans that N.Y.U. has made to faculty members and executives, but not about any individual loans.

Many of N.Y.U.’s loans help faculty members pay for apartments in the city. But since Mr. Grassley’s original request, The New York Times has reported that N.Y.U. also provides loans on vacation homes for some executives and star faculty members, including the Fire Island residence of its president, John Sexton, whose residence near Washington Square is also provided by the university.

Suffolk County land records indicate that one loan to Dr. Sexton of $137,000 may have been forgiven by N.Y.U., and another loan of $549,000 had its interest rate reduced last year to 0.19 percent. Other details of the loans to Dr. Sexton and other executives, including the original interest rates and terms of repayment, are not publicly available and the university has not disclosed them. The university has defended the loans as necessary to lure and retain executives and professors.

Photo

Treasury Secretary Jacob J. LewCredit
Alex Wong/Getty Images

“They promised my staff they’d be able to review loan documents related to other staff members” besides Mr. Lew, Mr. Grassley said. “The university and my staff confirmed a date to review those documents, but they canceled that review.” He added, “The only conclusion I can come to is any information they’d give us would be very embarrassing to them.”

Mr. Brewer, who is a lawyer, said to provide information about individual loans beyond what is legally required, “and to single out our faculty and our administration and staff with Senator Grassley’s staff having admitted that they’re not asking for that information from any other university, is not only a serious step for us to take regarding our obligations to our staff but it frankly holds those individuals out in a public way that is neither required nor called for.”

“They have had multiple responses, multiple rounds of information being provided to the senator’s staff in response to his questions as recently as the week before last,” he added. “After all that has been evaluated, if there are still additional inquiries, we’ve offered to get together with them, answer whatever questions they have.”

Representatives of Mr. Grassley’s staff are scheduled to meet on Thursday with N.Y.U.’s new legal representative in the matter, J. Steven Hart of the Washington law firm Williams & Jensen.

Mr. Grassley conceded that he has few practical resources for getting the materials he wants: as a Republican in a Democratic-controlled Senate, he has no power to subpoena documents. He said his inquiry was motivated not by partisanship but by the university’s tax status, noting that he had previously looked into spending at Harvard and Yale.

“They’re a nonprofit organization, they ought to act like a nonprofit organization,” he said. “They’ve got to justify their tax exemption.”

A version of this article appears in print on July 11, 2013, on page A24 of the New York edition with the headline: N.Y.U. Impeding Compensation Inquiry, Senator Says. Order Reprints|Today's Paper|Subscribe